Anti-reflective coating composition and coating film with excellent stain resistance

ABSTRACT

The present invention relates to a coating composition for forming an anti-reflective coating layer for a display device, comprising a fluorinated silane with low surface tension, a conductive polymer with antistatic properties, water, and a solvent. Thus, the coating film of the present invention prepared by coating the composition has high anti-reflection, excellent stain resistance to liquid-phase stains such as fingerprints and the solid-phase stains such as dust by controlling the refractive index, surface energy, and conductivity, and thus can be usefully applied to the outermost side of a display device, regardless of the type of substrates such as a Braun tube or a flat display film and the presence of other coating layers such as a hard coating layer and an anti-glare coating layer.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2004-0025704 filed on Apr. 14, 2004, which is herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coating composition for forming ananti-reflective coating layer with excellent stain resistance andanti-reflection. More specifically, the present invention provides acoating composition comprising a fluorinated silane with low surfacetension, a conductive polymer with anti-static property, and a solvent.

2. Description of the Related Art

There are various display devices, including Braun tubes such as cathoderay tubes (CRTs) for monitors and color picture tubes (CPTs) fortelevisions; and thin film transistor-liquid crystal displays(TFT-LCDs), PDPs, RPTSs, and LCDs for cellular phones, watches, andpicture display. Display devices which are exposed to natural lightreflect the light, and the reflected light causes eye fatigue andheadaches and lowers the contrast of the display devices by preventingimages from forming clearly at an inner part of the display.

To resolve the problems, a coating layer with low reflectance has beenresearched. Laid-open Japanese patent publication Nos. 1997-208898 and1996-122501 disclose a coating layer having a refractive index of1.28-1.38 by using a silicone compound and metal fluoride such asmagnesium fluoride (MgF₂). European patent publication No. 0 776 925 B1provides a coating film prepared by using a fluorinated silane and acompound including a fluorinated alkyl group, which obtains a coatingfilm with anti-reflectance and stain-resistance by decreasing surfaceenergy.

However, the coating layer with low reflectance including a fluorinatedcompound is easily charged on its surface by electrification such asthrough friction. Thus, dust can easily attach to the layer, and isdifficult to remove. In particular, because the layer with lowreflectance is usually located at an outermost part of the display, itmust have resistance to stains such as from fingerprints as well as fromdust. Stain-resistance of the reflective layer means that the electriccharge on the surface does not disappear or stop, but flows, and thuselectrostatic attraction for attaching the dust to the surface does notoccur.

Therefore, the prior art has disclosed methods of preventing dust fromattaching to the surface without static charge. Laid-open Japanesepatent publication No. 1994-65529 disclosed conductive metal fillerssuch as tin-doped indium oxide (ITO), antimony-doped tin oxide (ATO),and antimony-doped zinc oxide (AZO). However, the conductive metaloxides generally have a high refractive index, thus are not expected tohave an anti-reflective effect or to remove liquid-phase stains such asfingerprints.

Furthermore, in laid-open Japanese patent publication No. 2003-215306,the anti-reflection layer contains a fluorine group and a functionalgroup with a positive charge and a negative charge so as to improve theremoval of solid-phase stains such as dust and liquid-phase stains suchas fingerprints. However, it is susceptible to environmental change, andhas difficulty in removing the liquid-phase stains.

Therefore, it is needed to provide an anti-reflective coatingcomposition for display devices to provide anti-reflection propertiesand removal properties of both liquid-phase stains and dust, bycontrolling the refractive index, surface energy, and conductivity ofthe coating layer.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a coating compositionfor forming an anti-reflective coating layer in a display device, wherethe coating layer has anti-reflection properties caused by a lowrefractive index, and stain resistance caused by low surface energy andanti-static properties, and where the coating layer is applied to thefront side of the display device.

To accomplish the object, the present invention provides a coatingcomposition for forming an anti-reflective coating layer in a displaydevice which comprises (a) a sol-gel reaction product produced by aalkoxy silane, a fluorinated silane, a catalyst, water, and an organicsolvent; and (b) a conductive polymer.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The present invention is described in more detail below.

To endeavor to develop an anti-reflective coating composition forforming a coating layer in a display device, where the coating layer hasanti-reflection properties caused by a low refractive index and stainresistance caused by low surface energy and anti-static properties, andwhere the coating layer is applied to the front side of the displaydevice, the present inventor completed the present invention byinventing a coating composition comprising an alkoxy silane forproviding mechanical properties to a coating film, a fluorinated silanefor providing a low refractive index and excellent resistance toliquid-phase stains, and a conductive polymer for providing anti-staticproperties on the film surface to remove solid-phase stains such asdust.

The coating composition for forming an anti-reflective coating layer ina display device according to the present invention comprises:

-   -   (a) a sol-gel reaction product produced with (i) 0.3 to 15 parts        by weight of a alkoxy silane, (ii) 0.1 to 10 parts by weight of        a fluorinated silane, (iii) 0.001 to 0.5 parts by weight of a        catalyst, (iv) 0.01 to 10 parts by weight of water, and (v) 20        to 99.5 parts by weight of an organic solvent; and    -   (b) 0.02 to 50 parts by weight of a conductive polymer.

The alkoxy silane is contained in a coating composition to providemechanical strength to a coating film, so that the film can be appliedto the outermost side of the display device. Examples of the alkoxysilane are silicone compounds having four functional groups such astetramethoxysilane and tetraethoxysilane; and silicone compounds havingthree functional groups such as methyltrimethoxysilane,methyltriethoxylsilane, glycidoxypropyltrimethoxysilane,glycidoxypropyltriethoxysilane, etc.

If the amount of the alkoxy silane for producing the sol-gel reactionproduct is less than 0.3 parts by weight, it is difficult to maintainstrength of the coating layer for a display device. If the amount of thealkoxy silane exceeds 15 parts by weight, the coating layer can curl andhas difficulty in adhering to the substrate surface because of the highhardness of the coating layer.

The fluorinated silane is contained for lowering the refractive index,and for easily removing liquid-phase stains. Examples aretridecafluorooctyltriethoxysilane (DYNASYLAN F8261 ™, Degussa-Huls),heptadecafluorodecyltrimethoxylsilane (TSL8233™, Toshiba silicon,KBM-7803™, Shinetsu), heptadecafluorodecyltriisopropoxysilane(XC95-A9715™, Toshiba silicone), etc.

If the amount of fluorinated silane is less than 0.1 parts by weight, itis not sufficient to control the refractive index of the coating layer,or the stain resistance. If the amount is more than 10 parts by weight,the strength can fall.

In addition, the fluorinated silane is preferably used in an amount of10 to 50 wt % based on 100 wt % of the total amount of alkoxy silane andfluorinated silane. If the amount of fluorinated silane is less than 10wt %, a sufficient refractive index and stain resistance forliquid-phase stains cannot be obtained. If the amount exceeds 50 wt %,it can be difficult to control the molecular weight suitable for sol-gelreaction, and it decreases the strength of the film.

When the organic solvent is used at less than 20 parts by weight, ormore than 99.5 parts by weight, the composition cannot be easily coatedon a substrate surface.

Examples of the organic solvent are one or a mixture of at least twosolvents selected from the group consisting of alcohols, cellosolves,ketones, and mixtures thereof. The alcohols preferably include at leastone selected from the group consisting of methanol, ethanol, propanol,and butanol, but are not limited thereto. The cellosolves preferablyinclude at least one selected from the group consisting ofmethylcellosolve, ethylcellosolve, butylcellosolve, hexylcellosolve, andisopropoxycellosolve, but are not limited thereto. The ketonespreferably include at least one selected from the group consisting ofacetone, methylethylketone, diacetonealcohol, and methylisobutylketone.

Poor compatibility between fluorinated silane and the conductive polymercan induce phase separation in the coating composition. Thus, thecoating composition preferably includes an alcohol with a low boilingpoint (below 100° C.) as a solvent to prevent the phase separation. Morepreferably, the alcohol can be mixed with at least one solvent with amiddle boiling point (about 100° C. to 150° C.) such as one selectedfrom the group consisting of ketones and cellosolves. The amount ofsolvent with a middle boiling point in the mixture ranges from equal toor less than 30 wt %, and more preferably 1 to 20 wt % based on thetotal weight of the solvent mixture. If the amount of solvent with amiddle boiling point in the mixture is more than 30 wt %, phaseseparation between the fluorinated silane and conductive polymer canoccur, and it takes an excessively long time to dry the coatingsolution.

The catalyst is an acid such as nitric acid, hydrochloric acid, sulfuricacid, and acetic acid, and their metal salts. Examples of the salts area nitrate, a hydrochloride, a sulfate, or an acetate of zirconium orindium. The catalyst can be used in the amount of 0.001 to 0.5 parts byweight. If the amount of catalyst is less than 0.001 parts by weight, itis difficult to control the time of sol-gel reaction. If the amountexceeds 0.5 parts by weight, an increase of acidity of the coatingsolution cause corrosion.

The water is required for hydrolysis and condensation, and is used inthe amount of 0.01 to 10 parts by weight. If the amount is less than0.01 parts by weight, it is difficult to control the time of sol-gelreaction. If it exceeds 10 parts by weight, it is difficult to coat thecomposition.

The conductive polymer provides anti-static properties to the substratesurface so as to easily remove the solid-phase stains. Preferredconductive polymers include polythiophene resins such aspolyethylenedioxythiophene resin. In particular, to improveconductivity, the polythiophene resin can be doped with glutamic acid,alkylsufonic acid, anions of polystyrene sulfonic acid, and copolymersof styrene sulfonic acid and its anions. A conductive polymer which iscommercially available ispolyethylenedioxythiophene-polystyrenesulfonate, such as at least oneselected from the group consisting of P (Baytron P) (Bayer), PH, P HS, PHC, P ET V2, etc.

The conductive polymer is used in the amount of 0.02 parts by weight to50 parts by weight. If the amount is less than 0.02 parts by weight, itis difficult to remove solid-phase stains such as dust. If the amount ismore than 50 parts by weight, the strength of the coating film candecrease.

The coating composition according to the present invention can beprepared according to a method known to an ordinary person skilled inthe art. In an embodiment of the present invention, (a) (i) 0.3 to 15parts by weight of an alkoxy silane, (ii) 0.1 to 10 parts by weight of afluorinated silane, (iii) 0.001 to 0.5 parts by weight of a catalyst,(iv) 0.01 to 10 parts by weight of water, and (v) 20 to 99.5 parts byweight of an organic solvent are mixed and subjected to sol-gel reactionat pH 1 to 3, at a temperature of 15 to 90□, for 0.5 to 40 hours. Thesol-gel reaction is controlled to make the final reaction product have amolecular weight of 2000 to 20,000, which is advantageous for drying andhardening the coating film. Then, the conductive polymer at 0.02 to 50parts by weight is added to the sol-gel reaction product to prepare thecoating composition.

The solid content of the coating composition is preferably 0.5 to 30 wt%, and can decrease by further adding the organic solvent depending onthe temperature for drying or hardening.

The present invention provides a coating film prepared with the coatingcomposition. That is, the coating composition can be used as ananti-reflective coating agent for a display device, and can be appliedto a substrate, or a hard coating layer formed on the substrate, or ahigh refraction layer formed on the hard coating layer and substrate.Glass, plastic plate, or film can be used as the substrate. A coatingmethod is suitable selected depending on the kind of substrate. The hardcoating layer can be a UV curable resin, or a UV curable resin withinorganic nano-sized particles dispersed therein to improve wearresistance.

The coating composition can be coated on the substrate according to theconventional coating method. After coating, the processes of drying andhardening are preferably performed at a temperature of 70 to 120□, for10 to 40 hours.

In general, as the refractive index of an anti-reflective layer islowered, the beneficial properties of the anti-reflective layer areincreased. In particular, an increased difference in refractive indexbetween the anti-reflective coating layer and its lower layer increasesthe anti-reflection properties. The thickness of the anti-reflectivelayer is determined depending on the refractive index of the componentsand the wavelength of incident light. For example, if a hard coatinglayer with a refractive index of 1.51 and an anti-reflective layer witha refractive index of 1.38 are formed on the substrate, and thewavelength of incident light is 550 nm, the desired thickness of theanti-reflective layer is calculated to be 100 nm.

The anti-reflective coating layer can be applied to the outermost sideof a display device, regardless of the type of substrates used, such asa Braun Tube or a flat display film, or the presence of another coatinglayer such as a hard coating layer and an anti-glare coating layer.

The present invention is further explained in more detail with referenceto the following examples. These examples, however, should not beinterpreted as limiting the scope of the present invention in anymanner.

EXAMPLE 1

10 g of tetraethoxysilane, 3 g of heptadecafluorodecyltrimethoxylsilane,3 g of water, 0.12 g of hydrochloric acid, and 58.88 g of ethanol weremixed and sol-gel reacted at pH 2, at a temperature of 78□ for 3 hours.After the reaction product cooled to room temperature, 25 g of theconductive polymer Baytron P ET V2 was added thereto to produce ananti-reflective coating composition.

The coating composition was coated on a hard coating film by a rollcoating method, and dried to a thickness of 100 nm. The coated film washardened in an oven at 120□ for 10 hours.

EXAMPLE 2

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that the Baytron P ET V2 as aconductive polymer was substituted with 25 g of Baytron PH.

EXAMPLE 3

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that fluorinated silane wassubstituted for the heptadecafluorodecyltriisopropoxysilane.

EXAMPLE 4

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that the amount of ethanol and theconductive polymer were 83.86 g and 0.02 g, respectively.

EXAMPLE 5

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that the amount of ethanol andheptadecafluorodecyltrimethoxylsilane were 60.88 g and 1 g,respectively.

EXAMPLE 6

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that the amount of ethanol andheptadecafluorodecyltrimethoxylsilane were 51.88 g, and 10 g,respectively.

EXAMPLE 7

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that the solvent was substitutedwith a mixture of 35.28 g of ethanol, 11.8 g of diacetonealcohol, and11.8 g of butylcellosolve.

EXAMPLE 8

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that the solvent was substitutedwith a mixture of 78.1 g of ethanol, 0.39 g of diacetonealcohol, and0.39 g of butylcellosolve.

COMPARATIVE EXAMPLE 1

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that Baytron P ET V2 as aconductive polymer was not used.

COMPARATIVE EXAMPLE 2

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that theheptadecafluorodecyltrimethoxylsilane was substituted withmethyltriethoxylsilane.

COMPARATIVE EXAMPLE 3

The anti-reflective coating film was prepared substantially according tothe same method of Example 1, except that the amount of ethanol andBaytron P ET V2 were 68.88 g and 15 g, respectively.

TEST EXAMPLES

For the coating film prepared by Examples 1 to 8 and ComparativeExamples 1 to 3, the contact angle, removal capacity of a characterwritten with an oil-based pen, removal of dust, and reflectance wereevaluated.

1) Contact Angle of Water

The contact angle of water of the coating film was measured to determinethe property to readily remove a liquid-phase stain. When the contactangle is equal to or more than 90°, the liquid-phase stain was properlyremoved.

2) Removal Capacity of Character Written with Oil-Based Pen

The character written with an oil-based pen was observed with the nakedeye to determine the removal capacity.

3) Removal of Dust Capacity

The coating film was rubbed with a cotton cloth with width of 10 cm anda length of 10 cm 20 times, and was then sprayed with a spray powderfrom a distance of 30 cm, 5 times at intervals of one minute. After thepowder settled on the coating film, the film was blown with air at 2 atmfor 10 seconds, and then remaining powder was observed with the nakedeye to classify as “high” and “low”.

4) Reflectance

The back side of the coating film was treated with black, and thenreflectance was measured with a spectrophotometer (N&K) to determine theanti-reflection property depending on the minimum reflectance. TABLE 1Contact Character Dust Minimum Haze angle (°) removal removalreflectance (%) (%) Example 1 102 high high 2.0 0.7 Example 2 104 highhigh 1.8 0.6 Example 3 97 high high 2.2 0.6 Example 4 100 high low 1.60.6 Example 5 80 middle high 2.8 0.7 Example 6 104 high low 1.2 0.8Example 7 104 high high 1.8 0.4 Example 8 103 high high 2.0 0.6Comparative 104 high low 1.5 0.3 Example 1 Comparative 70 low high 3.30.7 Example 2 Comparative 99 high high 2.1 1.0 Example 3

In Table 1, the terms of ‘high’, ‘middle’, and ‘low’ in the column ofcharacter removal indicate removal properties of ‘above 90%’, ‘70% to90%’, and ‘below 70%’, respectively.

As shown in Table 1, the films coated with the coating compositionsincluding the conductive polymer had excellent dust removal properties,however the films coated with the coating compositions including asol-gel reaction product without a fluorinated silane had high surfaceenergy, high reflectance, and low removal capacity for liquid-phasestains.

1. A coating composition for forming an anti-reflective coating layer ina display device, comprising: (a) a sol-gel reaction product prepared byreacting (i) an alkoxy silane at 0.3 to 15 parts by weight, (ii) afluorinated silane at 0.1 to 10 parts by weight, (iii) a catalyst at0.001 to 0.5 parts by weight, (iv) water at 0.01 to 10 parts by weight,and (v) an organic solvent at 20 to 99.5 parts by weight; and (b) aconductive polymer at 0.02 to 50 parts by weight.
 2. The coatingcomposition according to claim 1, wherein the organic solvent is amixture comprising at least two solvents selected from the groupconsisting of alcohols, cellosolves, and ketones.
 3. The coatingcomposition according to claim 2, wherein the mixture comprisesalcohols, and further comprises at least one selected from the groupconsisting of ketones and cellosolves.
 4. The coating compositionaccording to claim 1, wherein the fluorinated silane is at least oneselected from the group consisting of tridecafluorooctylethoxysilane,heptadecafluorodecyltrimethoxylsilane,heptadecafluorodecyltriisopropoxysilane, and a mixture thereof.
 5. Thecoating composition according to claim 1, wherein the conductive polymeris a polythiophene.
 6. The coating composition according to claim 1,wherein the conductive polymer is a polythiophene doped with at leastone selected from the group consisting of glutamic acid, alkylsufonicacid, anions of polystyrene sulfonic acid, a copolymer of styrenesulfonic acid and its anions, and a mixture thereof.
 7. The coatingcomposition according to claim 1, wherein the catalyst is at least oneselected from the group consisting of acids, metal salts, and a mixturethereof.
 8. An anti-reflective coating film for a display deviceprepared with the anti-reflective coating composition according to claim1.